The present invention relates to an improvement in the method and apparatus for mixing fluids having different process variables such as concentration or temperature to obtain a fluid having a desired process variable.
Strong demand has arisen for so-called flexible process automation. Flexible process automation is defined as a process for preparing different items each having desired quality and quantity. In order to control the process, the response and controllability characteristics of the respective control systems must be improved.
Various types of mixture control operations such as supply air temperature control, supply air humidity control, turbidity control, or fuel calorie control are included in process control systems. The transient response and controllability characteristics of these control systems are largely influenced by changes in load such as production quantity, which, of course, detracts from the quality and uniformity of the resultant products. Thus, it is important to produce uniform, high quality products and also to use as little energy as possible.
FIG. 1 is a block diagram of a conventional mixture control apparatus. Referring to FIG. 1, solid lines represent an actual construction, and dotted lines represent signal flows, respectively. This mixture control apparatus mixes two fluids to obtain a desired, third fluid. More particularly, in this mixture control apparatus, a fluid A having a concentration Xa and flowing through a pipe 1 is mixed with a second fluid B having a concentration Xb and flowing through a second pipe 3 to prepare a third fluid C having a desired concentration Xs (but actually with a concentration Xc). The fluid C is supplied to a demand portion (not shown) through a third pipe 5.
The construction and operation of the mixture control apparatus will be described with reference to FIG. 1. In the following description, an output signal from a concentration detector 11 is not the concentration Xa but is a corresponding signal. However, for illustrative convenience, signals corresponding to respective physical quantities and calculated values are designated with the same reference numerals as those of the physical quantities and calculated values throughout the specification and in the accompanying drawings. The same reference symbols of concentration and flow rate of the fluids are also used in mathematical expressions.
The first flow detector 7 and the second flow detector 9 are arranged in the first and second pipes 1 and 3, respectively. The first and second flow detectors 7 and 9 detect flow rates Fa and Fb of the first and second fluids A and B and generate corresponding signals Fa and Fb, respectively. The first, second and third concentration detectors 11, 13 and 15 are arranged in the first, second and third pipes 1, 3 and 5, respectively. The first, second and third concentration detectors 11, 13 and 15 detect concentrations Xa, Xb and Xc of the first, second and third fluids A, B and C and generate corresponding signals Xa, Xb and Xc, respectively. A mixing ratio calculation circuit 17 receives a set point variable Xs of the concentration of the third fluid C and the signals Xa and Xb. The ratio calculation circuit 17 calculates the ratio K as a ratio of the flow rate Fa of the first fluid A to the flow rate Fb of the second fluid B, using the signals Xs, Xa and Xb. The circuit 17 then generates a corresponding signal K. The ratio K is calculated in the following manner. The overall material balance provides equation (1), and the component material balance provides equation (2). The ratio K is thus given by equation (3) below in accordance with equations (1) and (2). EQU Fa.times.K=Fb (1) ##EQU1## EQU K=(Xs-Xa)/(Xb-Xs) (3)
A first multiplier 19 receives the signals K and Fa and multiplies the signal K with the signal Fa. The first multiplier 19 then generates a corresponding signal KFa. The concentration control unit 21 receives the signals Xc and set point variable Xs and calculates the difference (Xs-Xc) between the concentration Xs and the actual concentration Xc of the fluid C. The unit 21 generates a corresponding concentration control signal .DELTA.X1. A second multiplier 23 multiplies the signal KFa with the concentration control signal .DELTA.X1 to obtain a set point variable Fs of the flow rate of the second fluid B. The second multiplier 23 then generates a corresponding signal Fs. The set point variable Fs of the flow rate of the fluid B is thus expressed by equation (4) below. A flow control unit 25 generates a control signal SC so as to set the flow rate Fb to the set point variable Fs in accordance with the signals Fs and Fb. The degree a flow control valve 27 opens is adjusted in response to the signal SC. The mixture control apparatus controls the mixing quantity of the second fluid to prepare the fluid C, having a desired concentration Xs. Signal .DELTA.X1 varies with respect to 1 as shown in equation (5). Substitution of equation (5) into equation (4) yields equation (6). EQU Fs=Fa.times.(Xs-Xa).times..DELTA.X1/(Xb-Xs) (4) EQU .DELTA.X1=1+.DELTA.C (5) EQU Fs=Fa.times.(Xs-Xa).times.(1+.DELTA.C)/(Xb-Xs) (6)
As is apparent from equation (6), the flow rate set point variable Fs of the second fluid B is calculated such that the ratio K is multiplied with the flow rate Fa of the first fluid A. For this reason, the conventional mixture control apparatus and the method thereof have the following drawbacks.
(1) The flow rate of one of the fluids to be mixed is controlled in accordance with the flow rate of the other thereof. For example, the set point variable of the flow rate of the second fluid B is calculated in accordance with the flow rate Fa of the first fluid A. For this reason, the set point variable Fs cannot accurately determine a transient change in the flow rate Fc (load flow rate) of the third fluid C. When the flow rate Fc in the mixture control apparatus changes, the set point variable Fs of the flow rate cannot be accurately calculated. In particular, for the accurate calculation of Fs, the flow rate Fb should be higher than the flow rate Fa. An accurate set point variable Fs can be calculated. For this reason, transient disturbance occurs in the conventional mixture control apparatus after the flow rate Fc changes. In other words, the conventional mixture control apparatus has low transient controllability.
(2) A gain correction value is obtained in accordance with a flow rate of one of the fluids to be mixed in a conventional mixture control apparatus. For example, a gain correction value GS of the control system in the conventional mixture control apparatus is given as equation (7) in accordance with equation (6). The gain correction value GS changes in accordance with a change in load flow rate Fc. However, in this case, the gain correction value GS is calculated in accordance with the flow rate Fa. The gain correction cannot be accurately performed after the flow rate Fc changes. The gain for controlling the concentration Xc of the third fluid C deviates from the optimal value, thereby degrading controllability. In other words, gain adaption for disturbance cannot be accurately performed. EQU GS=Fa.times.(Xs-Xa)/(Xb-Xs) (7)